This invention is directed to a process for preparing crystalline polyarylates which comprises: (a) forming a crystalline polyarylate prepolymer having a reduced viscosity of from 0.05 to about 0.4 dl/g by reacting at least one diester derivative of a dihydric phenol with at least one aromatic dicarboxylic acid in the presence of a solvent, at a temperature of from about 250.degree. to about 300.degree. C., and (b) heating the prepolymer so formed below its melting point to form a crystalline polyarylate having a reduced viscosity of from about 0.45 to about 1.2 dl/g.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane also identified as Bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative, which is then reacted with an aromatic dicarboxylic acid to form the polyarylate.
U.S. Pat. No. 3,684,766 issued Aug. 15, 1972, and U.S. Pat. No. 3,780,148 issued Dec. 18, 1973, describe a four step diacetate process for producing polyarylates. In the patented processes, in the first step a prepolymer is formed from, for example, a diacetate, such as Bis-phenol-A diacetate, and an aromatic acid, in the presence of a catalyst. In the second step, the prepolymer so formed is comminuted into small particles. In the third step, these particles are contacted with a crystallizing agent to crystallize the polyester. This may be accomplished by placing the particles in a fluidized bed reactor and contacting them with the crystallizing agent in the reactor. The patent states that the crystallizing step is necessary because it increases the melting point of the prepolymers so that the molecular weight can be built up to a satisfactory value without fusing or sticking of the particles. The fourth step involves heating the crystallized bisphenol polyester in the presence of an inert gas, optionally in a fluidized bed, at a temperature below the melting point and for a period of time which is sufficient to achieve an inherent viscosity of at least 0.5 in the final polymer.
The crystallization step described in U.S. Pat. Nos. 3,684,766 and 3,780,148 is a slow, time consuming and expensive operation.
In the process of this invention high molecular weight crystalline polyarylates are achieved without such a crystallization step.
U.S. Pat. No. 4,075,173 issued Feb. 21, 1978, describes the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-A, and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bisphenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2, of the patent: ##STR1##
This process is the diacetate process as described herein, or the "Acetate Process" as defined in the patent.
Column 2 of the patent states:
"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above-mentioned literature." PA1 "On the other hand, the route (1), Acetate process, is economically advantageous because the materials used are cheap and the operation is simple. For example, diacetate of bisphenol-A, a monomer for Acetate process, is synthesized by merely reacting acetic anhydride and bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate process, coloration and deterioration, are solved, Acetate process will become the most superior process."
Further, column 3 of the patent states:
Thus, the skilled workers in the field of polyarylate chemistry realize that the existing processes for producing polyarylates have one or more deficiencies, and that a need exists to develop a viable diacetate process for producing polyarylates.
In U.S. Pat. No. 4,075,173, a copolyester was prepared by the diacetate process by a solid-state polymerization of low molecular weight prepolymers without using crystallizing agents. The patentees state that their Acetate process is possible only when specific monomers are combined to form the prepolymer. These monomers are the diacetate of bisphenol-A, terephthalic acid and/or isophthalic acid and an acetate of p-hydroxybenzoic acid. The prepolymer is then converted to the desired high molecular weight polymer by solid state polymerization. Specifically, in the process of said patent, a prepolymer is first prepared by condensing the acetate of p-hydroxybenzoic acid, bisphenol-A diacetate with isophthalic and/or terephthalic acids at a temperature of from 200.degree. to 380.degree. C. for 1 to 10 hours. Catalysts capable of accelerating an ester exchange reaction can be used. The prepolymer is pulverized or pelletized and heated under reduced pressure or in an inert gas to convert it into a high molecular weight polymer.
Thus, as stated in U.S. Pat. No. 4,075,173, the diacetate processes for producing polyarylates are generally unsuitable since they are either economically unattractive and/or produce a commercially unmarketable product. The process of said U.S. Pat. No. 4,075,173 requires the use of p-hydroxybenzoic acid and produces a particular class of polyarylate copolymers. As previously stated, U.S. Pat. Nos. 3,684,766 and 3,780,148 require the use of crystallizing agents in a crystallizing step.
Therefore, a need exists for an economical and practical diacetate process for producing high molecular weight crystalline polyarylates.
U.S. Patent Application Ser. No. 069,818, filed Aug. 27, 1979 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether" describes a process for preparing polyarylates having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm, which process comprises reacting at least one diester derivative of a dihydric phenol with at least one aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the polyarylate produced, of a diphenyl ether compound, at a temperature of from about 260.degree. to about 350.degree. C.
U.S. Patent Application Ser. No. 070,039, filed Aug. 27, 1979 in the name of L. M. Maresca, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether Compound and A Catalyst" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of a diphenyl ether compound at a temperature of from about 260.degree. to about 350.degree. C. and in the presence of a magnesium catalyst.
U.S. Application Ser. No. 069,819, filed Aug. 27, 1979 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates of improved color which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260.degree. to about 350.degree. C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.
U.S. Patent Application Ser. No. 126,994, filed Mar. 3, 1980 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of at least one halogenated and/or etherated substituted aromatic or heteroaromatic compound, at a temperature of from about 260.degree. to about 350.degree. C.
In said U.S. Patent Application Ser. Nos. 069,818; 070,039; 069,819 and 126,994, supra, reaction of one or more dihydric phenols with an aromatic dicarboxylic acid produces polyarylate polymers or copolymers that essentially do not crystallize during the reaction and recovery steps.
The present process produces polyarylate prepolymers and high molecular weight polyarylate polymers and copolymers which are crystalline.
In the diacetate process for producing polyarylates, problems exist which must be economically and practically solved in order to have a viable, economically attractive process. One problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that sublimation of the diacid occurs. This disrupts the stoichiometry of the reaction and the polyarylate produced is not of acceptable molecular weight. To prevent sublimation of the diacid, several techniques have been developed. These include the use of large amounts of solvents together with a variety of catalysts and generally long reaction times in the polymerization process. However, these techniques are quite costly and do not provide an optimum process. Another problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that the viscosity of the system increases dramatically towards the end of the reaction and therefore the reaction becomes diffusion controlled (the molecules are not close enough to insure rapid reaction) rather than kinetically controlled. Also, the polymer product is difficult to handle (i.e., removal from the reactor) due to this high viscosity.
Yet another problem in the production of polyarylates by the diacetate process is that a carboxylic acid is a by-product of the reaction of a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid. In order to provide an efficient, economical process and a high molecular weight polyarylate, the acid, for example, acetic acid, has to be conveniently and efficiently removed.
It has now been discovered that crystalline polyarylates can be efficiently and economically produced by the process of this invention.
The utilization of a solvent in preparing a prepolymer in the process of this invention prevents sublimation of the aromatic dicarboxylic acid; thus producing polyarylates of acceptable molecular weight. Also, the solvent provides for better removal of the acetic acid by-product. Further, an additional benefit in using a solvent is that the viscosity of the system is decreased. This decrease in viscosity provides a faster reaction time since better mixing of the reactants occurs which allows the reaction to proceed under kinetic control.
Further, the presence of a solvent during the preparation of the prepolymer allows for the formation of crystallites. After a certain molecular weight is reached, spontaneous crystallization takes place. Thus, a free flowing highly crystalline powder containing the solvent is obtained. This crystalline powder is suitable for further polymerization in the solid phase by heating below its melting point. Therefore, the crystallization step as is required in the process of U.S. Pat. Nos. 3,684,766 and 3,780,148 is thus eliminated.
The unexpected findings of this invention are thus quite important. Typical melt polymerizations are indeed very difficult with high melting, high crystalline polyarylates because of the high temperatures required. Solution polymerizations are difficult due to the general insolubility of crystalline polymers. In contrast, the process of this invention allows easy formation of crystalline intermediates as well as highly crystalline, high melting polyarylates.